thanks you Orrie G
so if i look at the specs for the frame rails
3100 is 2.260 inch wide and 3600 is 2.250 so a 3/4 frame is .010 smaller.
3100 is 6.00 depth and a 3600 is 6.090 so a 3/4 frame is .090 taller
3100 is .140 thick and a 3600 is .190 thick so a 3/4 frame is .050 thicker
So i can confirm that my 1957 3600 frame is exact those spec.
But there is one thing i just notice :
is that a 1957 1/2 ton long box (3200)
as a 3/4 ton FRAME
So if somebody can just mesure is 3100 frame to confirm the manuel
it will be a very good info for future reference
thanks